The present invention relates to a process control technique and, more particularly, to a control method and control apparatus for Processing a Plurality of Controlled Variables which process a plurality of controlled variables.
Some of control systems each having a plurality of controlled variable measurement points and a plurality of control actuators have the first purpose of maintaining one controlled variable at a predetermined value as a whole, and the second purpose of maintaining a relation variable (controlled variable difference, controlled variable ratio, or the like) representing the relationship between a plurality of controlled variables.
For example, a temperature control system as shown in FIGS. 9A and 9B is proposed.
In the temperature control system of FIG. 9A, temperature sensors TS0, TS1, and TS2 serving as measurement points for a plurality of controlled variables (temperatures PV0, PV1, and PV2), and heaters H1 and H2 serving as a plurality of control actuators are arranged in a tank R. The first purpose of this temperature control system is to control the temperature PV0 of the center in the tank R to a predetermined set point SP by the heaters H1 and H2.
To only control the temperature PV0 of the center in the tank R to a predetermined value, outputs from the heaters H1 and H2 are not uniquely determined. In other words, the temperature PV0 of the center may be maintained at a predetermined value while the temperature PV1 near the heater H1 is higher than the temperature PV2 near the heater H2. To the contrary, the temperature PV0 of the center may be maintained at a predetermined value while the temperature PV1 near the heater H1 is lower than the temperature PV2 near the heater H2.
The temperature PV0 of the center in the tank R is controlled to a predetermined value, and the temperature distribution in the tank R is also important. That is, when the temperature difference between the temperature PV1 near the heater H1 in the tank R and the temperature PV2 near the heater H2 must be set to a predetermined value d12, outputs from the heaters H1 and H2 are uniquely determined.
However, outputs from the heaters H1 and H2 which are uniquely determined change depending on the temperature condition in the tank R. Even if the temperature difference between the temperatures PV1 and PV2 is controlled to a predetermined value, the temperature difference may be maintained at the predetermined value with a high heater output or a low heater output under the influence of heat insulation or disturbance in the tank R. Thus, any feedback control system must be constructed.
In the conventional control system, as shown in FIG. 9B, control calculation units PID1 and PID2 are respectively arranged for the heaters H1 and H2 to constitute independent feedback control systems using the heaters H1 and H2 as actuators for the temperatures PV1 and PV2 near the heaters H1 and H2. The control algorithm of the control calculation units PID1 and PID2 is PID control.
In FIG. 9B, the temperature PV0 of the center in the tank R is not substantially contained in the control system. The relationship between the temperatures PV0, PV1, and PV2 are investigated in advance.
The values of the temperatures PV1 and PV2 for making the temperature PV0 of the center in the tank R coincide with the predetermined set point SP and making the temperature difference between the temperatures PV1 and PV2 near the heaters coincide with the set point d12 are specified in advance on the basis of the investigation result.
In actual control, the control calculation unit PID1 controls the heater H1, and maintains the temperature PV1 near the heater H1 at a predetermined temperature. The control calculation unit PID2 controls the heater H2, and maintains the temperature PV2 near the heater H2 at a predetermined temperature.
Another example of the arrangement in FIG. 9B is a temperature control system in which the temperature PV0 of the center in the tank R is made to coincide with the predetermined set point SP and the ratio (PV1−Tmr)/(PV2−Tmr) of the rises of the temperatures PV1 and PV2 near the heaters from room temperature Tmr is made to coincide with the predetermined set point d12.
As an example other than the temperature control system, there is proposed a pressure control system which mixes two types of gases and supplies the gas mixture at a predetermined pressure, as shown in FIG. 10.
The pressure control system comprises a pressure sensor PS0 which measures a pressure PV0 of a mixture of two types of gases, gas flow control valves Val1 and Val2 serving as control actuators which control the flow rates of two types of gases, and flow sensors FS1 and FS2 which measure the gas flow rates PV1 and PV2 controlled by the valves Val1 and Val2.
The pressure control system has the first purpose of making the pressure PV0 coincide with the predetermined set point SP, and the second purpose of making the flow rate ratio PV1/PV2 of the flow rates PV1 and PV2 coincide with the predetermined set point d12.
The above-described control systems must uniquely decide outputs from control actuators to proper values in order to satisfy a specific relationship between the second controlled variables PV1 and PV2 in a situation in which the control actuators are redundantly arranged to control the first controlled variable PV0.
In the prior art, a control system which independently controls the second controlled variables PV1 and PV2 near control actuators is constituted, as described above. The relationship between the controlled variables PV0, PV1, and PV2 is investigated in advance to adjust the first controlled variable PV0.
The temperature control system of FIG. 9B in which the temperature PV0 of the center in the tank R is made to coincide with the predetermined set point SP and the temperature difference between the temperature PV1 near the heater H1 and the temperature PV2 near the heater H2 is made to coincide with the predetermined set point d12 performs independent control of respectively controlling the temperatures PV1 and PV2 near the heaters to specific values.
In this temperature control system, the temperature PV0 of the center which is the first purpose is not always accurately maintained at the predetermined set point SP. For example, when an unexpected heating substance or endothermic substance enters the tank R, the temperature PV0 of the center is not maintained at the set point SP.
In the temperature control system of FIG. 9B, the temperatures PV0, PV1, and PV2 must be investigated in advance. The number of steps before actual control is large, substantially wasting the time.
Further, the temperature control system cannot meet a request of adjusting, in accordance with importance, the priority levels of the first purpose of making the temperature PV0 of the center in the tank R accurately coincide with the predetermined set point SP and the second purpose of making the temperature difference between the temperatures PV1 and PV2 near the heaters coincide with the predetermined set point d12.
These problems also occur in the temperature control system in which the temperature PV0 of the center in the tank R is made to coincide with a predetermined set point SP and the ratio (PV1−Tmr)/(PV2−Tmr) of the rises of the temperatures PV1 and PV2 from room temperature Tmr is made to coincide with the predetermined set point d12, and the pressure control system of FIG. 10 in which the pressure PV0 is made to coincide with the predetermined set point SP and the flow rate ratio PV1/PV2 of the flow rates PV1 and PV2 is made to coincide with the predetermined set point d12.